Refrigeration systems have been used for many years of the type which use a compressor to drive a refrigerant through a closed-loop system. The compressor increases both the pressure and the temperature of the vaporous refrigerant before the refrigerant is directed into a condenser. As it passes through the condenser, the vaporous refrigerant is cooled and condensed to a liquid, while releasing heat to the surrounding environment, usually with the aid of a fan. The liquid refrigerant is now directed to a thermal expansion valve which provides a controlled release of the high pressure liquid refrigerant into a series of coils, commonly called an evaporator. As it passes through the thermal expansion valve, the liquid refrigerant undergoes a change of state from a high pressure liquid to a lower pressure vapor, while extracting thermal energy from the atmosphere surrounding the evaporator. The vaporous refrigerant is then drawn into the compressor to close the loop and to restart the process cycle.
When the outside air temperature falls below a certain temperature, many existing refrigeration systems cannot operate at a low enough condensing capacity without generating a condition known as liquid "hold-up" in the condenser. Liquid hold-up occurs when liquid refrigerant is backed up from the liquid receiver into the condenser, thus flooding a portion of the condenser with such liquid, thereby reducing the capacity of the condenser to transfer heat from the refrigeration system. This is very inefficient from an energy utilization standpoint, because unnecessary fans are running, excessive pressure drop occurs in the liquid line between the condenser and liquid receiver, and the compressor is working harder than necessary when this condition exists.
The compressor is typically driven by an electric motor and the major portion of system energy usage is incurred by the compressor's operation. It is important to keep the pressure at the outlet of the compressor sufficiently high to force the liquid refrigerant to remain in a liquid state in the refrigerant line between the condenser and the evaporator. If the outlet pressure is not sufficiently high at the compressor, then vaporous bubbles (called "flashgas") will form in the refrigerant line, thus reducing the overall system efficiency and cooling capacity since the thermal expansion valve (TEV) capacity is reduced when the refrigerant coming to it is in a partially vaporous state. The flashgas bubbles can be detected directly by an optical sensor, such as that disclosed in U.S. Pat. No. 4,644,755, by Esslinger et al.
In present refrigeration systems, the compressor outlet pressure is typically raised to the very high level sufficient to effectively cool the associated air spaces on the hottest day expected for that cooling season. This method of operation is, of course, not very efficient from an energy usage standpoint, since the compressor is continually consuming electrical energy at a rate that is calculated to properly work on the hottest day of that cooling season. On days where the outside ambient temperature is not as hot as the design temperature, such a refrigeration system is wasting a great amount of electrical energy.
A refrigeration system that has the capability to control the pressure in liquid refrigerant lines just above that required to maintain refrigerant in a liquid state could save electrical energy. The amount of energy saved would be the difference in the electrical energy utilized to drive the compressor hard enough to effectively cool the associated air spaces on the hottest day expected for that cooling season, and the electrical energy utilized to drive the compressor such that the pressure in the liquid refrigerant lines is controlled to a near optimal value. This energy savings would be significant, perhaps as much as fifteen percent (15%) of the entire electrical energy consumed by the refrigeration system.
A refrigeration control system that could perform the above energy savings and yet be retrofitted into existing refrigeration systems could save countless energy dollars without incurring the expense of installing entirely new refrigeration systems. If such a control system would be easily installed, then the expense of retrofitting the new refrigeration control system could be paid for quickly as the savings in energy usage occurs once the new system was put into operation.